This invention involves what would be considered a dark, neutral gray colored glass having a medium level visible light transmittance, i.e., in the range between 30 and 40 percent, a low ultraviolet radiation transmittance, and a low total solar energy transmittance. Although not limited to a particular use, the glass of this invention exhibits a combination of properties that make it highly desirable for use in architectural applications. These properties include a medium level visible light transmittance, low enough to reduce glare and yet high enough to allow adequate visibility therethrough, low total solar energy transmittance to reduce heat gain in the interior of the enclosure, low ultraviolet transmittance to reduce the adverse effects of ultraviolet radiation, a neutral gray color to facilitate coordination with a wide range of interior and exterior colors, and a composition compatible with conventional flat glass manufacturing methods. It is also considered to be an advantage of the invention that all compositions are nickel-free.
In the following discussion, certain terms well known to those skilled in the art are used to describe color in glass. Two terms or specifications for color, dominant wavelength and excitation purity, are derived from tristimulus values that have been adopted by the International Commission on Illumination. The numerical values of these two specifications for a given glass color can be determined by calculating the trichromatic coefficients, x, y and z, from the so-called tristimulus values (X,Y,Z) of that glass color. The trichromatic coefficients x and y then are plotted on a chromaticity diagram and numerically compared with the coordinates of Illuminant C, an established standard light source. (The trichromatic coefficient z value can be obtained by adding x and y and subtracting the total from 1.0) This comparison provides the color space position on the diagram to ascertain the excitation purity and dominant wavelength of the glass color.
Thus, a glass color may be specified either by its coefficients x and y or by its dominant wavelength and purity values. The lower the excitation purity of a color, the closer it is to the Illuminant C standard and the closer it is to being a so-called neutral color which does not distort the hues of objects seen through it.
An understanding of the foregoing terms and definitions thereof may be had by referring to the Handbook of Colorimetry prepared by the staff of the Color Measurement Laboratory, Massachusetts Institute of Technology. This book was printed in 1936 by the Technology Press, Massachusetts Institute of Technology, Cambridge, Mass. Also, a good explanation and list of definitions is given in Color in Business, Science and Industry, (3 Ed.) John Wiley & Sons (especially pages 170-72, 377-78). Useful also is An Introduction to Color, John Wiley & Sons (especially pages 105-106).
Those skilled in the art know that dominant wavelength, purity and light transmission all vary unpredictably with one another. Consequently, developing a new glass composition having a particular color, purity and light transmission value may be difficult. For example, an experimental change in the amount or relative proportions of one or more colorants in a glass composition intended to bring one of these numerical values closer to a target value may cause one or both the other values to drift off target.
In U.S. Pat. No. 3,296,004 to Duncan, neutral brown or bronze heat absorbing glasses are disclosed. Duncan expressly notes that the development of the particular color required a careful consideration of the transmittance characteristics of the glass and that the amounts of the colorants must be carefully controlled to achieve the desired color (dominant wavelength and excitation purity), transmittance and heat-absorbing characteristics. Thus, for example, Duncan points out that if his glass contained more cobalt oxide than he specifies, the color would be more blue than desired. Considering the glass composition of the present invention for a moment, however, the great unpredictability of this area of technology is well demonstrated by the fact that the present invention employs cobalt oxide in an amount well within the range used by Duncan, yet achieves a gray, not a brown or bronze color.
U.S. Pat. No. 3,723,142 to Kato discloses a neutral gray colored, heat absorbing glass having relatively high visible and solar transmittance values of from 57 to 63 percent, and a low excitation purity. The glass is defined as consisting essentially of the following base components in percent by weight: 68-75% SiO.sub.2, 0-5% Al.sub.2 O.sub.3, 5-15% CaO, 0-10% MgO, the sum of the CaO and the MgO being 6-15%, 10-18% Na.sub.2 O, 0-5% K.sub.2 O, the sum of the Na.sub.2 O and the K.sub.2 O being 10-20%, together with coloring components of 0.1-0.5% Fe.sub.2 O.sub.3, 0.003-0.02% Co.sub.3 O.sub.4, 0.0005-0.001% Se, and negligible NiO. The specification of the patent states that the quantity of NiO should be less than about 0.002% and preferably less than 0.0004%.
U.S. Pat. No. 4,866,010 to Boulos, et al. discloses a blue glass having a relatively high visible light transmittance of about 54 percent and a dominant wavelength of about 482 nm and a high color purity of about 13 percent. The glass contains from 0.3 to 0.6 percent Fe.sub.2 O.sub.3, 0.004 to 0.008 Co.sub.3 O.sub.4, and a relatively small amount of selenium in an amount of from 0.0001 to .001 percent. Again, the great unpredictability of this area of technology is well demonstrated by the fact that the present invention employs cobalt oxide in an amount well within the range used by Boulos, et al., yet achieves a gray, not a blue color.
U.S. Pat. No. 4,873,206 to Jones discloses a dark gray soda-lime silica glass having a low visible light transmittance of less than 20 percent, produced with a relatively high amount of iron (greater than 0.55 percent by weight), and a relatively high amount of selenium (greater than 0.003 percent). The glass product is suitable for use in sun roofs, but is too dark for many architectural applications requiring a higher visible light transmittance. In addition, because of the high volatility and consequently low retention rates of selenium in the glass (typically on the order of about 20 percent), a relatively large quantity of selenium must be added to the batch, much of which escapes to the atmosphere as a pollutant, in order to obtain the levels of selenium disclosed.
U.S. Pat. No. 5,023,210 to Krumwiede et al. also discloses a dark gray soda-lime silica glass having a low visible light transmittance of less than 20 percent, produced with from about 0.4 to, 0.7 percent by weight total iron, of which 15 to 20 percent is in the ferrous state, a relatively high amount of selenium, namely, greater than 0.003 weight percent, from 0.003 to 0.025 percent by weight CoO, and from 0.022 to 0.050 percent by weight Cr.sub.2 O.sub.3. It would be desirable to be able to make a dark, neutral gray, nickel-free glass, compatible with commercial flat, e.g., float, glass manufacturing techniques, having a medium level visible light transmittance between about 30 and 40 percent, and low ultraviolet radiation and total solar energy transmittance. In this respect, the above discussed dark grey glasses of the prior art are too dark for many architectural applications and indeed are generally only suitable for use in sun roofs.